Most modern cameras include automatic exposure systems which, to some degree, control the diaphragm opening or the time during which the shutters are open to result in good exposure with a preselected film. While some cameras have a manual override which permits the internal automatic exposure system to be bypassed, many cameras do not have such an override and the camera automatically selects the picture taking parameters as a function of the ambient light which is normally detected by a photo cell which is mounted for detecting light from the general direction of the subject.
There are numerous problems which result in the taking of photographs under abnormal light conditions, such as in astro-photography and micro-photography. One of the primary problems with existing automatic exposure systems of cameras is the relatively small sensitivity of the photo cells used in connection therewith. For example, when the quantity of light decreases any appreciable amount from a bright sun condition, the response from the photo cells becomes minimal and, so far as the exposure system is concerned, there is very little or no light present. Under this condition, the automatic exposure system automatically select the longest exposure time which the camera is designed to provide. For example, the maximum exposure time of the Polaroid SX-70 Land camera, is up to approximately 14 seconds. However, for some planetary photography as well as to achieve special effects, exposure times of less than 14 seconds may be desired although the ambient light conditions would normally result in such a long term exposure.
With respect to cameras which have a manual override, the very same problems normally arise. Thus, unless a photographer knows the lens opening and the exposure time which would give him the well-exposed photograph or special effect from experience, a photographer under these circumstances would normally have to rely on an exterior light meter for detecting the level of ambient light. Such light meters then provide the proper camera setting for manually controlled cameras. As with the built-in exposure control systems, the photo cells of the external light meters similarly must respond to the ambient light condition. When the amount of light is small, the light meters will similarly provide erroneous information for certain types of astro or micro-photography or for special effects.
Although not directed to the problems to which the present invention is directed, there is known an apparatus for photographing oscilloscope traces with conventional cameras. When taking a photograph of an oscilloscope trace, the exposure is sometimes determined by the ambient light and not by the brightness of the trace itself. The apparatus which is intended for use with oscilloscopes is disclosed in U.S. Pat. No. 3,696,720. However, the device disclosed in this patent is not sufficiently versatile to permit the same to be used in astro-photography, micro-photography or any photography under extremely adverse lighting conditions. Thus, the adjustment control on the prior art apparatus under discussion only provides sufficient range of exposure sufficient to photograph oscilloscope traces which may be made to have a relatively constant intensity by adjustment of the oscilloscope intensity controls. For this reason, the limited range of exposure which the prior art device provides is generally acceptable. Clearly, since the light intensity of the oscilloscope traces can always be adjusted, the prior art device primarily serves the function blocking out ambient light which may confuse the circuit of the internal photo control system of the camera and substitute for the ambient light a light source which is the equivalent or substitute for the light emanating from the oscilloscope screen.
As suggested above, with respect to astro-photography and micro-photography, the lighting conditions are frequently much more adverse and, accordingly, it is important that any exposure control adapter be more versatile than the prior art device so that, depending upon the lighting conditions and the desired effect, either a manual or automatic mode of operation may be utilized.